A new low magnetic field magnetar: The 2011 outburst of SwiftJ1822.3-1606

We report on the long-term X-ray monitoring with Swift, RXTE, Suzaku, Chandra, and XMM-Newton of the outburst of the newly discovered magnetar SwiftJ1822.3-1606(SGR1822-1606), from the first observations soon after the detection of the short X-ray bursts which led to its discovery, through the first stages of its outburst decay (covering the time span from 2011 July until the end of 2012 April). We also report on archival ROSAT observations which detected the source during its likely quiescent state, and on upper limits on SwiftJ1822.3-1606's radio-pulsed and optical emission during outburst, with the Green Bank Telescope and the Gran Telescopio Canarias, respectively. Our X-ray timing analysis finds the source rotating with a period of P = 8.43772016(2)s and a period derivative ss^-1, which implies an inferred dipolar surface magnetic field of B ≃ 2.7 × 10¹³G at the equator. This measurement makes SwiftJ1822.3-1606 the second lowest magnetic field magnetar (after SGR0418+5729). Following the flux and spectral evolution from the beginning of the outburst, we find that the flux decreased by about an order of magnitude, with a subtle softening of the spectrum, both typical of the outburst decay of magnetars. By modeling the secular thermal evolution of SwiftJ1822.3-1606, we find that the observed timing properties of the source, as well as its quiescent X-ray luminosity, can be reproduced if it was born with a poloidal and crustal toroidal fields of B_p ∼1.5 × 10 ¹⁴ G and B _tor ∼7 × 10¹⁴ G, respectively, and if its current age is 550kyr.